<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var a = 10;
        var b = 20;
        var c = null;
        var d;
        var e = true;
        var f = '字符串';
        console.log(a + b);   //30
        console.log(a - b);   //-10
        console.log(a * b);   //200
        console.log(a / b);   //0.5
        console.log(a % b);   //10
        console.log(a+c);     //10
        console.log(a+d);     //NaN
        console.log(a+e);     //11
        console.log(a+f);     //10字符串
    </script>
</body>

</html>